This invention relates to a ceramic component which is electrically conductive at low temperatures. More specifically, this invention relates to a ceramic component which is electrically conductive at low temperatures and which can be used for the preparation of electrodes suitable for use in the channel of the magnetohydrodynamic (MHD) generator or as the current leadout portion of an MHD electrode for use in a high-temperature MHD channel.
The environmental conditions within an operating MHD channel are very severe, and strenuous physical demands are placed on electrodes which must function in this environment. The plasma, which is an ionized gas or an inert gas seeded with as ionizing agent such as potassium, may reach temperatures up to 3000 K., while surface of the electrode may reach 2000 K. However, since the electrodes are attached directly to metal conductor frames which are generally of copper, the electrode-conductor temperatures can be no more than about 600-1000 K. Thus, the electrodes must be capable of withstanding a temperature differential between electrode-plasma interface and the electrode-conductor interface of up to about 1400 K. The electrodes must be able to withstand erosive forces from the plasma as it passes through the duct at near sonic velocities and they must either be protected from oxidation or be prepared from oxidation-resistant materials, since many plasmas, depending upon the particular fluid and its source, are oxidizing (P.sub.O.sbsb.2 =101 Pa) at operating temperature. The electrodes must also be able to withstand the highly corrosive effects of gaseous or molten potassium or coal slag when present in the plasma. The electrodes must be able to withstand the effects of electrochemical reactions which occur due to the passage of direct electric current through the anode and cathode in the presence of an electrolyte, i.e. the potassium seed or coal slag. Finally, the electrodes must be constructed of materials which are electrically conductive at the normal operating temperature of the channel and preferably at low temperatures and which can withstand the thermal shock of sudden temperature changes due to generator malfunction without the electrode separating from the channel or without electrode disintegration. Thus, it is a problem to find materials from which electrodes can be made which can withstand the rigors of such an environment.
Most of the materials which are best able to withstand the rigors of an operating MHD channel are generally ceramic-type in nature. These materials include zironia and hafnia stabilized with various oxides such as ceria, yttria or terbia, spinel doped with iron or chromium and yttria and lanthanum-chromite doped with strontium and magnesia. While all of these materials have adequate electrical conductivity at MHD operating temperatures, most of them, particularly the hafnia and zirconium based materials, have very low electrical conductivity at the lower temperatures ranging from room temperature up to 1200 to 1300 K. The low electrical conductivity or high resistivity of these materials at the lower temperatures is detrimental to the electrical performance and channel efficiencies. For example, the passage of large electric currents through these materials with low electric conductivity results in ohmic heating, higher voltages and possible decomposition. The voltages required to push the current through the resistive areas of the electrode decrease the electrical efficiencies of the MHD generator. The higher voltages may also cause electrochemical degradation of the material and ultimately leads to premature destruction of the electrodes. Attempts to solve these problem have led to the use of metal inserts in the ceramic electrodes as a low-temperature current leadout or to the use of a composite electrode consisting of a metal lower current leadout portion topped with a ceramic capable of withstanding the MHD environment. This solution has caused other problems such as (1) cracking, separation or spalling, due in large measure to the difference in the coefficient of expansion between the metal and ceramic, (2) electrochemical interactions between the metal and ceramic, and (3) increased difficulty and cost of manufacture.